Timing synchronization for networks with radio links

ABSTRACT

Precision Timing Protocol (PTP) related functions for use in packet communications carried in part by a microwave communications link include setting of time of day values across the microwave link and providing transparent clock functions. The PTP functions may be used for synchronizing radio base stations in a cellular network. The transparent clock can bridge Ethernet switches associated with microwave stations providing the microwave communications link.

BACKGROUND OF THE INVENTION

The present invention relates generally to timing synchronization incommunication systems and, more particularly, to timing synchronizationfor communication systems including radio communication links.

It is often advantageous in a communication network for devices in thenetwork to have a common time base. For example, in 3G mobile/cellularcommunication systems, radio base stations (RBS) are synchronized towithin a few microseconds. Precise synchronization facilitates efficientcommunication such as for allocation of time slots among devices andinterference reduction. The electronics industry has developed severalstandard protocols for use in synchronizing clocks, for example, thePrecision Time Protocol (PTP) of IEEE Std. 1588. PTP includes sendingtiming-related messages between nodes in a communication network. Thetiming-related messages include, for example, a first node transmittingover a link a time-stamped packet to supply its time base to a secondnode, and the second node transmitting, over the link, a packet to thefirst node requesting and thereafter receiving a reply with the time ofreceipt of the request packet, so that the second node may haveinformation regarding packet time of travel over the link. With thesetiming related messages the second node may be able to synchronize itslocal clock to the clock time of the first node, with it often beingassumed that packet travel time over the link between the nodes isconstant.

Mobile communication networks often use radio links, for example atmicrowave or millimeter-wave frequencies to provide backhaul from radiobase stations to a base station controller or gateway reached through acarrier network. Delivering high-accuracy synchronization usingpacket-based techniques such as IEEE Std. 1588 (2008), incorporated byreference herein, is especially challenging over radio links. Radiosystems have progressed in both capacity and cost. However, many systemsmay use proprietary communication protocols. The protocols may notinclude features for establishing common time bases. Moreover, radiosystems may have variable delays in propagating information, for exampledue to changes in bitrate and modulation scheme used for transmissionover the wireless radio link. In addition, wireless backhaul equipmentmay include switches, for example Ethernet switches, and data passingthrough such switches may encounter variable delays. Thus, timingsynchronization of mobile communication systems connected by microwaveradio links may be difficult.

BRIEF SUMMARY OF THE INVENTION

In some aspects the invention provides a communication device in datacommunication with another communication device over a radio link, thecommunication device comprising: circuitry including packet handlingcircuitry configured to send and receive packets, to determine anexpected time of day (TOD) value for a future time, and to provide theTOD value for transmission to the other communication device; and radiolink communication circuitry configured to receive the TOD value andtransmit the TOD value over the radio link to the other communicationdevice.

Another aspect of the invention provides a method of performing timingrelated processing in a packet-based network communications over amicrowave communications link, the method comprising: synchronizingclocks of a first packet handling circuitry element and a second packethandling element separated by a microwave communications link, the firstpacket handling circuitry element being part of a first microwavestation and the second packet handling circuitry element being part of asecond microwave station, by determining, by a one of the first packethandling circuitry element and the second packet handling circuitryelement, an expected time of day (TOD) value for a time in the future atwhich a periodic signal is to be commonly received or generated by thefirst packet handling circuitry element and the second packet handlingcircuitry element, and transmitting the expected TOD value over themicrowave communications link to the other of the first packet handlingcircuitry element or the second packet handling circuitry element;determining a time of arrival of a packet at the first packet handlingcircuitry element; transmitting the time of arrival of the packet at thefirst packet handling circuitry element over the microwavecommunications link to the second packet handling circuitry element;determining a time of departure of the packet from the second packethandling circuitry element; and setting a correction field of the packetto a value based on at least the time of departure of the packet fromthe second packet handling circuitry element and the time of arrival ofthe packet at the first packet handling circuitry element.

Another aspect of the invention provides a method of performing timingrelated processing in a packet-based network communications over amicrowave communications link, the method comprising: synchronizing aclock of a first Ethernet switch with a clock of a second Ethernetswitch separated by a microwave communications link, the first Ethernetswitch being part of a first microwave station and the Ethernet switchbeing part of a second microwave station; determining a time of arrivalof a packet at the first Ethernet switch using a first time of day (TOD)domain; determining a time of departure of the packet from the firstEthernet switch using a time of day (TOD) from a first TOD domain anddetermining a time of departure of the packet from the first Ethernetswitch using a TOD from a second TOD domain; setting a correction fieldof the packet to a value based on of least the time of departure of thepacket from the first Ethernet switch using the TOD from the first TODdomain and the time of arrival of the packet at the first Ethernetswitch; and transmitting the packet, including the time of departure ofthe packet from the first Ethernet switch using the TOD from the secondTOD domain, over the microwave communications link to the secondEthernet switch.

Another aspect of the invention provides a method of performing timingrelated processing in a packet-based network communications over amicrowave communications link, the method comprising: receiving, by thefirst packet handling circuitry element, information regarding amodulation and forward error correction (FEC) scheme used by circuitryperforming microwave communications link functions associated with thefirst packet handling circuitry; receiving, by the first packet handlingcircuitry, information regarding a modulation and forward errorcorrection (FEC) scheme used by circuitry performing microwavecommunications link functions associated with the second packet handlingcircuitry; shaping bandwidth of the information of packets provided tothe circuitry performing microwave communications link functionsassociated with the first packet handling circuitry element to match adata bandwidth of the circuitry performing microwave communications linkfunctions associated with the first packet handling circuitry elementbased on the modulation and FEC scheme; performing, by the first packethandling circuitry element, at least portions of a precision timingprotocol synchronization process with the second packet handlingcircuitry element, the process additionally including discarding anyprecision timing protocol packets received within any of a plurality ofpredefined time periods of a change in the information regarding themodulation and FEC scheme used by circuitry performing microwavecommunications link functions associated with the second packet handlingcircuitry.

Another aspect of the invention provides an Ethernet switch, comprising:a plurality of ingress ports and egress ports; a switch fabricconfigurable to route packets between ingress ports and egress ports;and circuitry configured to determine a first time of arrival for afirst packet at a one of the ingress ports in a first time of day (TOD)domain, and to determine a first time of departure for the first packetat a one of the egress ports in the first TOD domain and to determine asecond time of departure for the first packet at the one of the egressports in a second TOD domain.

Another aspect of the invention provides an Ethernet switch, theEthernet switch coupled to microwave communications circuitry, theEthernet switch comprising: a switch fabric coupling input ports andoutput ports, at least one of the input ports and at least one of theoutput ports coupled to the microwave communications circuitry; a shaperto shape bandwidth of data passed from the switch fabric to themicrowave communications circuitry; and processing circuitry configuredto receive information regarding a transmission modulation and forwarderror correction (FEC) scheme in use by the microwave communicationscircuitry for transmitted information over a microwave communicationslink and to provide information to the shaper as to a bandwidth to beused in shaping bandwidth, the processing circuitry being furtherconfigured to receive a reception modulation and FEC scheme in use bythe microwave communications circuitry for received information over themicrowave communications link and to command dropping of a precisiontiming protocol packet received within a predefined period of time of achange in the reception modulation and FEC scheme.

These and other aspects of the invention are more fully comprehended onreview of this disclosure.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a diagram of a communication system including radiocommunication links in accordance with aspects of the invention;

FIG. 2 is a block diagram of a communication device in accordance withaspects of the invention;

FIG. 3 is a flowchart of a synchronization process in accordance withaspects of the invention;

FIG. 4 is a flowchart of a communication process in accordance withaspects of the invention;

FIG. 5 is a block diagram of a further communication system inaccordance with aspects of the invention;

FIG. 6 is a flowchart of a communication process in accordance withaspects of the invention;

FIG. 7 is a block diagram of a further communication system inaccordance with aspects of the invention;

FIG. 8 is a flowchart of further synchronization processes in accordancewith aspects of the invention;

FIG. 9 is a flowchart of a communication process in accordance withaspects of the invention;

FIG. 10 is a semi-block diagram, semi-process flow of a furthersynchronization system and process in accordance with aspects of theinvention.

DETAILED DESCRIPTION

FIG. 1 is a diagram of an example communication system including radiocommunication links in accordance with aspects of the invention. Thecommunication system generally includes radio base stations forproviding communications with mobile subscribers, for examplesubscribers using cellular communications devices, with the radio basestations coupled to a carrier network by communications links thatinclude radio communication links. In the example communication systemof FIG. 1, radio base stations 107, 117 with respective antennas 109,119 are in communication with the subscribers. Wireless backhaulequipment 101, 111 passes information from the radio base stations toand from, for example, carrier network equipment 103. The carriernetwork equipment may be part of a wired carrier network used forcarrying subscriber communications, and in that regard the carriernetwork equipment may be viewed as a link to or a node on a wiredcarrier network.

As shown in FIG. 1, the communication system includes a first wirelessbackhaul station 101 in communication with a second wireless backhaulstation 111 over a radio link, for example a microwave link. The firststation is, for example, co-located with and directly cabled to amobile/cellular radio base station (RBS) 107, and the second station is,for example, co-located with and directly cabled to another RBS 117. Thefirst station 101 includes an antenna 105 that sends and receivesdigitally coded microwave signals to and from another antenna 115included at the second station 111. The first station is connected tothe carrier network equipment 103 by a wired link, and the secondstation is coupled to the carrier network equipment via the radio linkto the first station 101.

The portions of the wireless backhaul stations performing the microwavecommunication functions may not utilize a time base in common with othernetwork components, with the microwave communicating portions of thebackhaul stations for example utilizing their own time base. It isgenerally preferable, however, that the RBS 107 and 117, separated bythe microwave link, have synchronized clocks.

The RBS 107 and the RBS 117 each include a local clock. synchronized toa clock of the carrier network equipment 103. The RBS 107 maysynchronize its local clock to the clock of the carrier networkequipment using signaling received by way of the first station, with thesignaling for example conforming to IEEE 1588v2. For example, ifcommunications between the carrier network equipment and RBS 107 flowthrough an Ethernet switch of the backhaul station 101, that portion ofthe backhaul station may perform as a transparent clock. As indicated inFIG. 1, the carrier network equipment serves as a master clock and theRBS 107 is a slave clock, although the node serving the master clockwill generally be determined in accordance with a best master clockalgorithm.

The RBS 117 also synchronizes its local clock to the clock of thecarrier network equipment, for example in accordance with IEEE 1588v2.Communications between the first backhaul station and the secondbackhaul station, to which the RBS 117 is coupled, flow through theportions of the backhaul stations performing microwave communicationfunctions. The signaling for synchronization of the local clock of RBS117, therefore, is provided over the microwave link between the backhaulstations.

The local clock time of day (TOD) for the first station may be set, forexample, using a precision timing protocol (PTP) flow that is providedby network elements, such as the carrier network equipment 103, coupledto the first station. In overview the TOD for the second station may beset by the first station transmitting a TOD value to the second station,with the TOD value sent to the second station being a value that will bethe TOD of the first station, and to which the second station may setits local clock, at the next pulse or transition of a periodic signalcommonly received or synchronously generated by both stations.

In some embodiments the commonly received or synchronously generatedsignal is a pulse-per-second (1PPS) signal. For example, as shown inFIG. 1 the first station 101 and the second station 111 each alsoreceive a pulse-per-second (1PPS) signal. The 1PPS signals arrive at, orare generated by, the stations at substantially the same time instancesor with a substantially fixed and known time offset. In one embodiment,the 1PPS signals pulse at a rate of one pulse per second, although otherrates may also be used. The 1PPS signals may be, for example, providedover the microwave link or a by a separate mechanism such as from globalpositioning system devices.

In some embodiments each of the microwave stations include packethandling circuitry, for example an Ethernet switch. Each Ethernet switchmay, for example, receive packets input to its corresponding station byway of wired connections and provide those packets for communicationover wired or wireless connections. For example, the packets may beprovided to an RBS coupled to the station by a wired connection, or thepackets may be provided to other station elements for further processingand microwave transmission to the other station. Packets received by theother station would be provided to that station's Ethernet switch forfurther communication over further wired or wireless connections.

In various embodiments the TOD value is generated by an Ethernet switchof one station, for example the first station, and received by the otherstation, for example the second station, with the second station usingthe 1PPS signal, for example generated by a portion of the stationinvolved with microwave communications functions, to determine the timeat which the TOD applies.

In some embodiments, the Ethernet switches may operate so as to form atransparent clock from a wired ingress port of one Ethernet switch,across the microwave link, and to a wired egress port of the otherEthernet switch. The transparent clock may be, for example, a PTPtransparent clock or a transparent clock in accordance with IEEEStandard 1588 (2008). As the transparent clock bridges both the Ethernetswitches and the microwave communicating link, the transparent clock maybe considered a distributed transparent clock.

For example, it may be assumed that the first station is transmittinginformation of the packets over a microwave link to the second station,although the converse may also apply. In such a scenario the Ethernetswitch of the first station may set a time stamp to a time of arrival ofa packet at an ingress part of the Ethernet switch, with the packet,including the time stamp, thereafter forwarded by the first station tothe second station. The time stamp, for example, may be placed in areserved field of a packet header, or otherwise placed in the packet.The Ethernet switch of the second station forwards the packet over thewired connection, with the Ethernet switch of the second station settinga time stamp field of the packet using the second station's time, whichis synchronized with the first station's time, for example using the TODvalue and the 1PPS signal. In some embodiments the time stamp field is acorrection field of the packet, with the value of the correction fieldbeing based on, and in many cases the difference between, an expectedtime of egress of the packet from the second station or the Ethernetswitch of the second station and the time of arrival time stamp set bythe first station.

The embodiment of FIG. 1 additionally includes a third wireless backhaulstation 121 in communication with a fourth wireless backhaul station131, with the fourth station co-located with yet another RBS 137 whichcommunicates with mobile subscribers by way of antenna 139. The thirdstation 121 is in wired communication with the second station 111, andthe second and third stations may be located in proximity to oneanother, and the fourth station may also communicate with anothercomponent via wired connections 133.

In operation, the third and fourth stations may synchronize their localclocks as discussed with respect to the first and second stations, withthe second station, for purposes of clock synchronization, performingthe function of providing a clock time that was performed by the carriernetwork equipment with respect to a clock time for the first station,and the third station providing a TOD value for use by the fourthstation. Similarly, the third and fourth stations may also includeEthernet switches, and third and fourth stations may set time stamps andcorrection fields as discussed with respect to the first and secondstations.

FIG. 2 is a block diagram of a communication device in accordance withaspects of the invention. The communication device may be used, forexample, as, at least in part, the master, transparent clock, or slavestation of the communication system of FIG. 1. The communication devicereceives and transmits microwave signals via an antenna 205. The antenna205 is coupled to a microwave radio system 203, which includes amicrowave radio frequency block 207 and a microwave modem block 209. Themicrowave radio frequency block 207 may include, for example, a poweramplifier in the transmit path and a low noise amplifier in the receivepath. The microwave radio frequency block 207 also includes anupconverter in the transmit path and a downconverter in the receivepath. In some embodiments, the microwave radio frequency block 207 isconfigured as an outdoor unit (ODU). The microwave radio frequency block207 is coupled to the microwave modem block 209, which may containintermediate frequency (IF) and/or baseband processing functions. Themicrowave modem block 209 supplies a signalfor transmission to themicrowave radio frequency block 207, and the microwave radio frequencyblock 207 supplies the received signalto the microwave modem block 209.The microwave modem block 209 converts the received modulated signal todigital form, including, for example, demodulating and decoding thesignal. The microwave modem block 209 also converts an unmodulateddigital signal for transmission from digital form, including, forexample, coding and modulating the signal.

The digital signals of the microwave radio system are coupled to anEthernet switch 201. The Ethernet switch generally includes physicallayer devices (PHYs) 221, 229 for sending and receiving signals to andfrom the microwave radio system and to and from a wired interface, aswitch 227 for routing signals, and, in various embodiments, a processor231 or other logic elements for control of device operations. The devicemay also include a clock 241, although the clock may be external to thedevice in some embodiments and as shown in FIG. 2. Similarly in variousembodiments some or all of the PHYs, switch, and/or processor may beimplemented as separate devices, and may include varying additionalcomponents. The PHYs, which may be Ethernet PHYs, convert digitalsignals to packets of information, and vice versa. The first PHY 221communicates with the microwave radio system, and the second PHY 229communicates to and from other communication devices over a wirednetwork (which may include one or more RBS and microwave radio systems).Between the PHYs, the packets are communicated through the switch block227, which routes the packets. In the embodiment illustrated in FIG. 2,the communication device includes two physical layer devices; however,in other embodiments, there may be a greater number of physical layerdevices with the packets routed by the switch block between the physicallayer devices, for example, accordingly to addresses contained in thepackets. Some packets, in addition or instead, may be routed to and fromthe processor 231, which may be programmable.

The local clock 241 provides a time base for the device. The local clock241 may operate, for example, as a master clock or as a slave clock,depending on, for example, an outcome of a best master clocknegotiation. In some embodiments, the physical layer devices processtiming related messages that they receive. For example, a physical layerdevice may parse packets to determine what type, if any, timing relatedmessages are in the packets. Depending on the type of message, thephysical layer device may modify the packet to signal the time at whichthe packet is received on transmitted.

The communication device may signal the value of its local clock by thecommunication device transmitting the value via the antenna 205. Forexample, the programmable processor 231 in the communication device maycreate a control message packet signaling the value the local clock willhave at a predetermined time in the future. In more detail, the localclock 241 may be timing locked to a IPPS signal with the value signaledin the control message packet being the value the local clock 241 willhave at the next pulse of the IPPS signal. In some embodiments, theprogrammable processor 231 may create the packet with the controlmessage packet with the first physical layer device 221 placing thevalue in the packet.

Alternatively, the communication device may receive, via the antenna205, a signal indicating the value to which the local clock should beset at the predetermined time in the future. For example, theprogrammable processor 231 may receive, decode, and execute a controlmessage packet signaling the value for the local clock at thepredetermined time in the future. In more detail, the local clock 241may be timing locked to a 1PPS signal with the value signaled in thecontrol message packet being the value for the local clock 241 at thenext pulse of the 1PPS signal. In some embodiments, the packet with thecontrol message packet may be processed at least partially by the firstphysical layer device 221.

FIG. 3 is a flowchart of a synchronization process in accordance withaspects of the invention. The process provides for timingsynchronization from a first device to a second device. Thesynchronization process may be performed in the microwave communicationsystem of FIG. 1. Additionally, part of the process may be performed bythe communication device of FIG. 2. Various parts of the process may beperformed by specific circuitry of the communication device, such as aphysical layer device, or by a programmable processor according toprogram instructions.

In block 301, the first device receives a pulse of a IPPS signal. Inresponse, or based upon the occurrence of some other predefined event orevents, the first device determines the value its local clock will haveat the next pulse of the 1PPS signal. For example, when the 1PPS signalpulses once per second, the process adds one second to the current valueof the local clock. In block 311, the process transmits the value thefirst device's local clock will have at the next pulse of the 1PPSsignal to the second device. In some embodiments, the process transmitsthe value in an Ethernet packet containing a control message. In thesecond device, the process receives, decodes, and extracts the clockvalue from the message.

In block 321, the process receives a pulse of a 1PPS signal at thesecond device. Although the 1PPS signals at the first and second devicesare synchronized, due at least to delay in communicating the time valuein block 311, the pulse of the 1PPS signal at the second device is the“next pulse” of the 1PPS signal for which the clock value was determinedby the master device in block 301. In block 331, the process sets thelocal clock in the slave device to the clock value received by thesecond device in block 311.

In some systems, the synchronization process is performed at regularintervals. In other systems, the second device transmits messagessignaling the value of its local clock to the first device. The firstdevice may then initiate the synchronization process when the value ofthe second device's local clock does not match the local clock of thefirst device.

FIG. 4 is a flow diagram of a process for processing packets includingtime protocol related information that are communicated over a microwavelink. The process may be performed, for example, by the first and secondstations of FIG. 1, each of which for example may include thecommunication device of FIG. 2.

In block 411 a precision time protocol packet is received at an ingressport of an Ethernet switch. In block 413 the Ethernet switch timestampsthe packet with a time of arrival of the packet at an ingress port ofthe switch. In various embodiments the Ethernet switch may timestamp thepacket by writing a time of arrival in a timestamp field of the packet,or by writing a time of arrival in some other field of the packet, or bywriting some other time in accordance with precision time protocolsusing the time of arrival. The timestamp may be determined, in someembodiments, at some time after arrival of the packet, with the Ethernetswitch accounting for processing delays within the Ethernet switch fromthe time of arrival to the time of determination of time of arrival. Insome embodiments the time stamp may be determined by a PHY within theEthernet switch, and in some embodiments the time stamp may bedetermined by logic circuitry, including a processor, of the Ethernetswitch.

In block 415 the process formats the packet into a non-precision timeprotocol packet, although in some embodiments operations of block 415are skipped or optional. The information of the time of arrival isretained in information of the packet. Formatting the packet into anon-precision time protocol packet may be useful, for example, if thepacket is to be transmitted, received, or otherwise processed by networkelements not configured or expecting to process precision time protocolpackets. In block 417 the process transmits the packet to anothernetwork node, for example by way of a microwave modem and RF unit, whichin various embodiments may be a microwave outdoor unit (ODU). In someembodiments, however, the microwave ODU may only include the RF unit,with the microwave modem being part of an indoor unit (IDU), which mayinclude the Ethernet switch, or the Ethernet switch, the microwavemodem, and the RF unit may all be part of an ODU.

In block 419 the packet is received by the other node. The other nodemay receive the packet, for example, using a microwave RF unit andmicrowave modem. In block 421 the process optionally, (for example ifoperations of block 415 have been or were to have been performed),reformats the packet into a precision time protocol packet, and thereformatting may be performed, for example, by an Ethernet switch of theother node. In block 423 the process performs a time code operation onthe packet using an expected egress time of the packet from the othernode or, more particularly, from an egress port of the Ethernet switch.The time code operation may be performed by logic circuitry, including aprocessor, of the Ethernet switch, or by a PHY of or associated with theEthernet switch. The egress time may be determined prior to egress ofthe packet from the Ethernet switch or PHY, with the Ethernet switch orPHY accounting for expected delays between the time of determination andthe expected time of egress.

In some embodiments the time code operation is to set a correction fieldof the packet to a value indicating the time period from ingress of thepacket into the Ethernet switch of the transmitting node to egress ofthe packet from the Ethernet switch of the receiving node. The value,therefore, indicates a time period, which may vary from packet topacket, of processing by both Ethernet switches and processing andtransmission by both microwave modems, in effect making the combinedEthernet and microwave processing systems a transparent clock forprecision time protocol purposes.

FIG. 5 is a block diagram of a further communication system inaccordance with aspects of the invention. In FIG. 5 a first microwavestation 511 is in communication with a second microwave station 513 viaa microwave communications link, with each of the microwave stationsalso in communication with other network nodes via wired communicationslink, including fiber optic lines.

The microwave stations each have a local clock synchronized to theother, for example by way of the method discussed with respect to FIG.3.

The first microwave station receives precision time protocol packetsover a wired link, with the packets received by a precision timeprotocol capable PHY 515, for example an IEEE 1588v2 compliant PHY. ThePHY determines a value for a time of ingress of a packet into the PHY,using the local clock of the station, and inserts the value into a timefield of the packet. The value may be determined at some point in timeafter the packet is received by the PHY, with the

PHY accounting for time spent processing the packet between time ofreception and time of determining the value. The packet is passed to aswitch unit 517 of the first microwave station. The switch formats thepacket into a time unaware format, retaining the time value within thepacket, and routes the packet. A modem and RF unit 519 of the firstmicrowave station receives the formatted packet, processes the packetfor microwave transmission, and transmits information of the formattedpacket over a microwave link to the second microwave station.

A modem and RF unit 521 of the second microwave station receivesinformation of the formatted packet and performs microwave receptionprocessing, for example amplification of the received signal,downconversion to baseband, and forward error correction processing. Theformatted packet is received by a switch unit 523 of the secondmicrowave station, which reformats the packet into a time aware format,for example a precision time protocol in accordance with the IEEE 1588(2008) standard, (1588 v2) and routes the reformatted packet to a PHY525 of the second microwave station. The PHY 525, like the PHY 515, is aprecision time protocol capable PHY, for example an IEEE 1588v2compliant PHY. The PHY determines a value for a time of egress of thepacket from the PHY, using a local clock of the station. In someembodiments the time of egress is written into a field of the packet. Inseveral embodiments, however, the PHY determines a correction value forthe packet, with the correction value being, for example, the time ofegress from the PHY 525 minus the time of ingress into the PHY 515. Thecorrection value may be written into a correction field of the packet,directly in some embodiments but more commonly in additive manner towhatever value may already be present in the correction field. Similarto the PHY 515, the PHY 525 may determine the time of egress of thepacket prior to actual egress of the packet, with the PHY accounting forprocessing time between time of determination of egress time and actualegress time.

FIG. 6 is a flow chart of a communication process in accordance withaspects of the invention. The process may be performed, for example, bythe microwave stations of FIG. 5, and portions of the process may beseparately performed by the first microwave station of FIG. 5 and secondmicrowave station of FIG. 5.

In block 611 the process synchronizes time protocol capable devices oftwo microwave stations. The time protocol capable devices may bephysical layer devices, and the physical layer devices (PHYs) may be atingress and/or egress points for electrical communications for thestations. In some embodiments the physical layer devices are compliantwith 1588v2 protocols. The devices may be synchronized, for examplethrough use of the method discussed with respect to FIG. 3 or elsewhereherein.

In block 613 the process determines and writes a time stamp value into areceived packet, with the time stamp value preferably indicating a timeof arrival of the packet at a first microwave station. The time stampvalue may be determined by a PHY of the first microwave station. Inblock 615 the packet is processed by the first microwave station, forexample by reformatting the packet into another format. For example, thereceived packet may be in a time aware protocol, for example a precisiontime protocol or a 1588v2 protocol, and the packet may be formatted intoa time unaware protocol. In some embodiments the packet is formattedinto another protocol through an encapsulation scheme, or by splittingthe packet into two or more packets, which may include information fromother packets as well, or by adding additional control signalinginformation, or placing control signaling information into a payload ofthe packet. In block 617 the process transmits information of the packetover a microwave link. In various embodiments the operation of block 617include adding forward error correction information to information ofthe packet, modulating the packet for microwave transmission,upconverting to microwave frequencies, and amplifying the resultantsignal for transmission through a microwave antenna.

In block 619 the process receives the transmitted information, with thereception occurring for example by a second microwave station. Inreceiving the transmitted information the process will generally amplifya received signal, downconvert the signal to baseband, recover digitalinformation of the downcoverted signal, and possibly correct thedigitial information using the forward error correction information. Inblock 621 the process processes the digital information, for example byreformatting the information into a time aware protocol and/or routingthe information to a physical layer device. In block 623 the processdetermines and writes a clock adjustment value into a field of thepacket. The clock adjustment value may be determined by the PHY of thesecond microwave station that was synchronized with the PHY of the firstmicrowave station. The clock adjustment value may be, for example, thedifference between an expected egress time of the packet from the PHY ofthe second microwave station and the timestamp value determined by thePHY of the first microwave station, and in some embodiments additionallywhatever value may already be present in the field of the packet intowhich the clock adjustment value is to be written.

FIG. 7 is a block diagram of a further system in accordance with aspectsof the invention. The system includes a first microwave station 711 incommunication with a second microwave station 713 over a microwave link.The first microwave station includes a switch 715 in data communicationwith a microwave modem and RF unit 717. The switch unit may be forexample an Ethernet switch, and the switch unit includes a processor. Alocal clock 719 provides time information to the switch. The microwavemodem and RF unit, which generally includes baseband processingfunctions and RF functions, communicates over a microwave airwave linkwith the second microwave station. The second microwave stationsimilarly includes a switch 721, a microwave modem 723, and a localclock 725.

Processors of the switch units synchronize their clocks, with in someembodiments one of the switch units serving as a master clock and one ofthe switch units serving as a slave clock, with the choice of masterclock and slave clock determined for example using a best master clockalgorithm. Synchronization may occur through a variety of manners, asfor example discussed herein.

In some embodiments the switch units provide data to the microwavemodems at a rate such that the microwave modems may operate with aconstant delay of data through the microwave modems. In suchembodiments, for example, the switch units may include shapers that mayadjust bandwidth of data provided to the modems, with the shaperssetting the bandwidth such that the microwave modems have a fixed delayin data passage through the modems. In some embodiments bandwidth of themicrowave modems is dependent on transmission modulation and forwarderror correction (FEC) schemes utilized by the microwave modems, whichmay depend on a variety of factors including atmospheric conditions,interfering transmissions, and other factors. In such embodiments theswitches receive information from their respective modems as to themodulation and FEC schemes, and adjust shaper bandwidth accordingly, forexample to match operating bandwidth of the microwave modems.

With known delays through the microwave modems, in some embodimentsprocessors of the switches may synchronize their clocks through use oftransmission of a time stamped packet from a first of the switch unitsto a second of the switch units, transmission of a request for a time ofreceipt packet from the second switch unit to the first switch unit, anda reply transmission from the first switch unit. As it is possible thatthe modems may change their modulation and FEC schemes, preferablysynchronization packets that bridge such a change are discarded withoutbeing used, with synchronization re-performed after any change in modemmodulation and FEC scheme changes.

FIG. 9 is a flow diagram of a communication process in accordance withaspects of the invention. The process may be performed, for example, bya switch unit of the microwave stations of FIG. 7.

In block 913 the switch unit receives modulation and FEC schemeinformation from a microwave modem associated with the switch.Preferably the modulation and FEC scheme information is provided bothfor data to be transmitted by the modem and for data received by themodem, in the event that different schemes are used depending on thedirection of communication. The switch unit may receive the information,in various embodiments, at periodic intervals, for example through apolling process or periodic signaling by the modem, or by the modemsending the switch unit signals upon scheme changes.

In block 915 the switch unit adjusts bandwidth of data provided to themodem for transmission. The bandwidth is adjusted such that the modemreceives data at a bandwidth no greater than the modem's maximum databandwidth for the current modulation and FEC scheme. In some embodimentsthe bandwidth is adjusted such that the modem receives data at abandwidth equal to the modem's maximum data bandwidth for the currentmodulation and FEC scheme.

In block 917 the switch unit determines if the modem has changed itsmodulation and/or FEC scheme. If not, the switch unit continues toprovide the modem data at the rate determined in block 915. If themodulation and/or FEC scheme has changed, however, the switch unit inblock 919 discards any precision time protocol packets or frames whoselatency through the microwave link might be different than expected dueto the change in modulation and/or FEC scheme. The process thereafterreturns to block 913 to re-obtain modem modulation and FEC schemeinformation, and continue the process.

FIG. 8 is a flow diagram of a synchronization process for two devicesseparated by a microwave link in accordance with aspects of theinvention. The process may be performed, for example, by devices in themicrowave stations of FIG. 1, or the stations or processors of theswitches of FIG. 7.

In block 811 the process determines a synchronization method. Themethods include microwave modem synchronization, side channelsynchronization, per modulation scheme synchronization, known latencyskew synchronization, or alternative synchronization. In variousembodiments determination of the synchronization method is determined atsystem installation, by other components, or based upon systeminformation provided by the microwave modem.

In some embodiments microwave modems provide a synchronization mechanismto synchronize microwave modems across a microwave link. In suchembodiments the process may continue to block 813, with the switch unitreceiving a synchronization pulse and associated time from itscorresponding modem. The process in block 815 sets the local clock timeto the value provided by the modem, and thereafter returns.

In some embodiments the microwave link includes a low bandwidth sidechannel, for example for transmission of microwave modem controlsignals, with the side channel having a known latency. In suchembodiments the process may continue to block 817, with the switch unitsending and receiving synchronization and delay response messages inorder to determine synchronization information. The process in block 819sets the local clock to the value determined as a result of the messagessent and received in block 817, and thereafter returns.

In some embodiments the microwave modems may have, or be able to beforced to have, fixed and known latency for transmission and receptionfor each of its modulation and FEC schemes. In such embodiments theprocess may continue to block 821. In block 821 the switch unitdetermines the modulation and FEC scheme in use by the modem, preferablyfor both transmission and reception, provides the modem data at a rateno greater than a maximum allowed data rate of the modem, and transmitsand receives synchronization and delay response messages in order todetermine synchronization information. The process in block 821 sets thelocal clock to the value determined as a result of the messages sent andreceived in block 823, and thereafter returns.

In some embodiments the microwave modems may have a symmetrical or knownlatency skew. In such embodiments the process may continue to block 825,with the switch unit sending and receiving synchronization packets, withthe switch in block 827 performing a PTP filtering algorithm todetermine a clock time. In block 829 the process sets the local clock tothe value determined by the PTP filtering algorithm, and thereafterreturns.

In some embodiments an alternative synchronization is used. In suchembodiments the process may continue to block 831, with the switch unitutilizing a synchronous Ethernet (syncE) channel, if supported by themodems, or IPPS processing to determine a clock time. In block 833 theprocess sets the local clock time to the time determined in block 831,and thereafter returns.

FIG. 10 is a semi-block diagram, semi-process diagram of a furthersystem and process in accordance with aspects of the invention. In FIG.10, a first microwave station 1011 is in communication with a secondmicrowave station 1013. The first microwave station includes a microwavemodern/RF unit 1019 for providing microwave communications, and a switchunit 1012 including a switch 1017 for routing packets to and from a port1018 coupled to the microwave modem/RF unit and various other ports, ofwhich only one, port 1015, is illustrated in FIG. 10. The switchgenerally operates under control of a processor or processing circuitry(not shown in FIG. 10). In some embodiments the microwave modem/RF unitand the switch unit are located in a single housing as, for example, anODU, while in some embodiments the switch unit and the microwavemodem/RF unit are separately housed.

Similarly, the second microwave station 1013 includes a microwavemodem/RF unit 1021 for providing microwave communications, and a switchunit 1014 including a switch 1023 for routing packets to and from a port1022 coupled to the microwave modem/RF unit and various other ports, ofwhich, as with the first microwave station, only a single port 1025 isillustrated in FIG. 10. As with the first station, in some embodimentsthe microwave modem/RF unit and the switch unit are located in a singlehousing as, for example, an ODU, while in some embodiments the switchunit and the microwave modem/RF unit are separately housed.

In operation, the switch unit of the first microwave station may receivea precision time protocol packet at port 1015, with the switch unitdetermining a time of arrival TSarr1 of the packet at the switch unit,with the switch unit utilizing its TOD in determining TSarr1. The switchunit writes the time of arrival TSarr1 into a field of the packet. Thetime of arrival TSarr1 is, in some embodiments, written to reservedbytes of the PTP packet. The switch unit routes the packet through itsswitch, in this case towards port 1018 coupled to the microwave modem/RFunit, although the packet may be routed to other ports, for example aport with a wired connection to an RBS associated with the station.

Prior to sending the packet to the microwave modem/RF unit, or to, forexample, an RBS coupled by a wired connection to the first microwavestation, the switch unit determines a time or times of departure for thepacket from the switch unit. The times of departure include a time ofdeparture TSdep1s utilizing the switch unit's TOD and a time ofdeparture TSdep1m utilizing a TOD of the microwave modem/RF unit, whichthe microwave modem/RF unit provides to the switch unit. In someembodiments the time of departure TSdep1m need not be determined, forexample if the packet is not to be routed to the microwave modem/RFunit. The switch unit, in some embodiments, sets a correction field ofthe packet to a value equal to the current value of the correction fieldplus the quantity TSdep1s minus TSarr1, or CF=CF+(TSdep1s−TSarr1), with(TSdep1s−TSarr1) representing in various embodiments a switch latencytime for the packet.

In several embodiments the microwave modem/RF unit may have a differentTOD than the switch unit. For example, in various embodiments themicrowave modem/RF units of both the first station and the secondstation may share a common TOD. Accordingly, the switch unit also, insome embodiments, writes the value of TSdep1m to a reserved field of thepacket.

The microwave modem/RF unit of the first microwave station performsmicrowave transmission operations on the packet information, includingtransmitting the packet to the second microwave station. The microwavemodem/RF unit of the second microwave station receives the packetinformation and performs associated microwave reception operations, andprovides the packet to port 1022 of the switch unit 1014. The switchunit 1014 determines times of arrival to the switch unit for the packet,including a time of arrival TSarr2m utilizing the microwave modem/RFunit's TOD and a time of arrival TSarr2s utilizing the switch unit'sTOD.

The switch unit updates the correction field of the packet by settingthe correction field of the packet to a value equal to the current valueof the correction field plus the quantity TSarr2m minus TSdep1m, orCF=CF+(TSarr2m−TSdep1m), with (TSarr2m−TSdep1m) representing in variousembodiments a microwave system communications latency time. The switchunit also writes the value of TSarr2s into the packet, for example inreserved bytes of the packet.

The switch unit 1014 routes the packet through its switch, for exampletowards port 1025, from which the packet will depart from the switchunit. Prior to the packet departing from the switch unit, the switchunit determines an expected time of departure TSdep2 for the packet. Insome embodiments the switch unit sets the correction field of the packetto a value equal to the current value of the correction field plus thequantity TSdep2 minus TSarr2s, or CF=CF+(TSdep2−TSarr2s), with(TSdep2−TSarr2s) representing in various embodiments a switch latencytime for the packet. The correction field therefore includes, upondeparture from the switch unit, information as to both switch unitlatency time, for both switches, and microwave system communicationslatency time. Each of these components of latency separately determined,with the latencies determined in different time domains for the switchand the microwave system communication. In operation, therefore, atransparent clock is provided, which may be considered a concatenateddistributed transparent clock, as the transparent clock crosses time ofday domains and is a sum of multiple calculated latencies.

Accordingly, aspects of the invention provide for synchronization ofdevices separated by a microwave link, and implementation of adistributed transparent clock across a microwave link. Although theinvention has been discussed with respect to varying embodiments, itshould be recognized that the invention includes the novel andnon-obvious claims supported by this disclosure.

1.-13. (canceled)
 14. A method of performing timing related processingin a packet-based network communications over a communications link, themethod comprising: synchronizing a clock of a first Ethernet switch witha clock of a second Ethernet switch separated by a microwavecommunications link, the first Ethernet switch being part of a firstmicrowave station and the Ethernet switch being part of a secondmicrowave station; determining a time of arrival of a packet at thefirst Ethernet switch using a first time of day (TOD) domain;determining a time of departure of the packet from the first Ethernetswitch using a time of day (TOD) from a first TOD domain and determininga time of departure of the packet from the first Ethernet switch using aTOD from a second TOD domain; setting a correction field of the packetto a value based on at least the time of departure of the packet fromthe first Ethernet switch using the TOD from the first TOD domain andthe time of arrival of the packet at the first Ethernet switch; andtransmitting the packet, including the time of departure of the packetfrom the first Ethernet switch using the TOD from the second TOD domain,over the microwave communications link to the second Ethernet switch.15. The method of claim 14, wherein the first TOD domain and the secondTOD domain are independent of one another.
 16. The method of claim 14,wherein the first TOD domain is a TOD domain for the first and secondEthernet switches.
 17. (canceled)
 18. The method of claim 14, whereinthe first TOD domain is a TOD domain for the first and second Ethernetswitches and the second TOD domain is a TOD domain for circuitryperforming microwave communication link functions.
 19. The method ofclaim 18, further comprising further comprising determining a time ofarrival of the packet at the second Ethernet switch using a TOD from thefirst TOD domain, determining a time of arrival of the packet at thesecond Ethernet switch using a TOD from the second TOD domain, anddetermining a time of departure of the packet from the second Ethernetswitch using a TOD from the first TOD domain.
 20. The method of claim19, wherein setting the correction field of the packet to the valuebased on at least the time of departure of the packet from the firstEthernet switch using the TOD from the first TOD domain and the time ofarrival of the packet at the first Ethernet switch using the TOD fromthe first TOD domain comprises setting the correction field of thepacket to the value based on at least the time of departure of thepacket from the second packet handling circuitry element using the TODfrom the first TOD domain, the time of arrival of the packet at thesecond Ethernet switch using the TOD from the first TOD domain, the timeof arrival of the packet at the second Ethernet switch using the TODfrom the second TOD domain, the time of departure of the packet from thefirst Ethernet switch using the TOD from the second TOD domain, and thetime of arrival of the packet at the first Ethernet switch using the TODfrom the first TOD domain.
 21. The method of claim 20, wherein settingthe correction field of the packet to the value comprises setting thecorrection field to a value equal to a value of the correction fieldplus the time of departure of the packet from the second Ethernet switchminus the time of arrival of the packet at the second Ethernet switchusing the TOD from the first TOD domain plus the time of arrival of thepacket at the second Ethernet switch using the TOD from the second TODdomain minus the time of departure of the packet from the first Ethernetswitch using the TOD from the second TOD domain plus the time ofdeparture of the packet from the first Ethernet switch using the TODfrom the first TOD domain minus the time of arrival of the packet at thefirst Ethernet switch.
 22. The method of claim 14, wherein synchronizingclocks of a first Ethernet switch and a second Ethernet switch separatedby a microwave communications link device is performed by way oftransmission and reception of precision time protocol messagestransmitted and received by way of a microwave side channel of themicrowave communications link.
 23. A method of performing timing relatedprocessing in a packet-based network communications over a microwavecommunications link, the method comprising: receiving, by the firstpacket handling circuitry element, information regarding a modulationand forward error correction (FEC) scheme used by circuitry performingmicrowave communications link functions associated with the first packethandling circuitry; receiving, by the first packet handling circuitry,information regarding a modulation and forward error correction (FEC)scheme used by circuitry performing microwave communications linkfunctions associated with the second packet handling circuitry; shapingbandwidth of the information of packets provided to the circuitryperforming microwave communications link functions associated with thefirst packet handling circuitry element to match a data bandwidth of thecircuitry performing microwave communications link functions associatedwith the first packet handling circuitry element based on the modulationand FEC scheme; and performing, by the first packet handling circuitryelement, at least portions of a precision timing protocolsynchronization process with the second packet handling circuitryelement, the process additionally including discarding any precisiontiming protocol packets received within any of a plurality of predefinedtime periods of a change in the information regarding the modulation andFEC scheme used by circuitry performing microwave communications linkfunctions associated with the second packet handling circuitry.
 24. AnEthernet switch, comprising: a plurality of ingress ports and egressports; a switch fabric configurable to route packets between ingressports and egress ports; and circuitry configured to determine a firsttime of arrival for a first packet at a one of the ingress ports in afirst time of day (TOD) domain, and to determine a first time ofdeparture for the first packet at a one of the egress ports in the firstTOD domain and to determine a second time of departure for the firstpacket at the one of the egress ports in a second TOD domain.
 25. TheEthernet switch of claim 24, wherein the circuitry is further configuredto determine a second time of arrival for a second packet at another oneof the ingress ports in the first TOD domain and to determine a thirdtime of arrival for the second packet at the another one of the ingressports in the second TOD domain, and to determine a third time ofdeparture for the second packet at another one of the egress ports inthe first TOD domain.
 26. The Ethernet switch of claim 24, wherein thecircuitry is further configured to adjust a correction field of thefirst packet using a difference between the first time of departure andthe first time of arrival.
 27. The Ethernet switch of claim 26, whereinthe circuitry is further configured to place the second time ofdeparture in a reserved field of the first packet.
 28. The Ethernetswitch of claim 25, wherein the circuitry is further configured toadjust a correction field of the second packet using a differencebetween the third time of arrival and a value in a reserved field of thesecond packet.
 29. The Ethernet switch of claim 28, wherein thecircuitry is further configured to adjust the correction field of thesecond packet using a difference between the second time of arrival andthe third time of departure.
 30. An Ethernet switch, the Ethernet switchcoupled to microwave communications circuitry, the Ethernet switchcomprising: a switch fabric coupling input ports and output ports, atleast one of the input ports and at least one of the output portscoupled to the microwave communications circuitry; a shaper to shapebandwidth of data passed from the switch fabric to the microwavecommunications circuitry; and processing circuitry configured to receiveinformation regarding a transmission modulation and forward errorcorrection (FEC) scheme in use by the microwave communications circuitryfor transmitted information over a microwave communications link and toprovide information to the shaper as to a bandwidth to be used inshaping bandwidth, the processing circuitry being further configured toreceive a reception modulation and FEC scheme in use by the microwavecommunications circuitry for received information over the microwavecommunications link and to command dropping of a precision timingprotocol packet received within a predefined period of time of a changein the reception modulation and FEC scheme.
 31. The Ethernet switch ofclaim 30, wherein the transmission modulation and FEC scheme and thereception modulation and FEC scheme are the same modulation and FECscheme.
 32. The method of claim 14, wherein the communications link is amicrowave communications link.
 33. The method of claim 32, wherein thesecond TOD domain is a TOD domain for circuitry performing microwavecommunication link functions.